Permanent-magnet rotor for electric motors and process of manufacturing the same

Abstract: Permanent-magnet rotor for electric motors including at least two semitubular permanent magnets (10) which are at least partially embedded in a core made of two-component epoxy resin or plastics. The core is obtained in a mold (20) by molding in place or pouring over a pre-molded casing that contains the magnets, or by pouring so as to form, together with said core, a containment casing (12).

Description: The present invention relates to a permanent-magnet rotor for electric motors, and to a method of making a permanent-magnet rotor. As is known, electric motors with permanent-magnet rotors are currently used in the field of small centrifugal pumps. The magnets that are used are produced by sintering appropriate powders under the action of a magnetic field. For motors having rotors with an axial size of up to approximately 3 cm, the magnets are manufactured with a tubular shape by filling an appropriate mold with the powders and by applying axial compression. The compression is such that the axial dimension of the finished part is approximately one third of the dimension occupied by the powders when mold filling ends. For electric motors with a rotor having greater axial dimensions it becomes practically impossible to obtain the rotor with the above described method in an economically and qualitatively satisfactory manner, and therefore it is preferred to manufacture semitubular parts to be assembled so as to compose the tubular rotor. These semitubular parts are again obtained by sintering powders in a magnetic field, but in this case compression occurs at right angles to the axial direction. For rotor assembly, initially the two semitubular permanent magnets used to be enclosed in a tubular casing made of stainless steel which, being a diamagnetic material, in theory has no effect on the magnetic field produced by the stator of the motor. However, practical tests have shown that a dielectric current forms in the air gap between the magnets and the pole pieces due to the presence of the stainless steel casing and of the wall of the rotor chamber, causing a 30% drop in efficiency. An attempt was later made to embed the two semitubular permanent magnets in an injection-molded plastic casing, but even this attempt failed to yield the expected results due to the fact that the necessary minimum thicknesses of the plastic material and the significant axial size of the rotor, combined with the rather low fluidity of the injection-molded material, prevent the complete filling of the intended regions and thus give an unacceptable final result. Increase in injection pressures to achieve complete filling caused cracking of the magnets due to their great hardness and consequent great fragility. Also known from EP-A- 0218477 is a magnet rotor assembly as defined in the preamble of claim 1, which discloses a permanent-magnetic rotor for an a.c. synchronous electric motor, comprising at least one pair of diametrically opposed poles mounted on a rotor shaft. The or each pair of poles is constituted by two individual pole members and are held in diametrically opposed and preferably non-abutting relationship by one or more non-magnetic, preferably plastic, holding members. More particularly such prior EP-A-0218477 discloses a rotor which comprises as holding members an inner core and an outer casing both of non magnetic material and it furthermore comprises two outer cap members, which can be considered as outer flanges. Such prior art patent did not offer a solution for keeping in place the axial shaft and the permanent magnet members so as to render possible an in-situ forming, by pouring the molding material, the rotor assembly, neither did it provide a solution for permanently securing the cap members to the assembly, through the very action of the molding material, nor did it suggest means to fill the spaces through epoxy resin so as to ensure a stable permanent arrangement of the poles and the shaft forming such rotor assembly. From EP-A-0459355 a permanent magnet rotor assembly is known, which has a cylindrical core member, a plurality of arcuate permanent magnet segments disposed around an outer periphery of the core member and a non-magnetic sleeve disposed around outer peripheries of the arcuate permanent magnet segments. The non magnetic sleeve is made of material formed in a die and entering the assembly through holes in an upper flange member. Such prior art patent failed to teach how the spatial relationship between shaft and permanent magnet elements can be kept into a stable condition since it did not provide an area, to be filled in by molded material, between shaft and permanent magnetic elements, such filling being the only means which is able to compensate for any irregularities and which is able to resist any stress developed by the rotating rotor, during rotation. The prior art comprises furthermore Patent No. GB-A-2191638 which describes a magnet assembly, for use with a flywheel alternator, comprising a plurality of permanent magnets of generally rectilinear cross-section, the magnets being held within a recess and spaced apart from each other by hollow cylindrical rubber spacers. The magnets are initially assembled in a jig and then fitted into the recess under compression; epoxy resin is used to secure them in place. DE-A-3102959 further describes a permanent magnetic rotor for electric motors comprising semitubular permanent magnets embedded in a core of plastics obtained by pouring over a pre-molded cup-shaped containment casing, the magnets being rigidly coupled to the cup-shaped casing by the plastic core. A principal aim of the present invention is therefore to provide a permanent-magnet rotor of the type with semitubular magnets so as to eliminate the above described drawbacks encountered in previous attempts. A consequent important object is to provide a rotor the structure whereof has no effect on the operation of the motor. Another object is to provide a process that allows to mass-produce the rotor at low costs. Another object is to provide a process that allows to obtain high-quality rotors for electric motors. With this aim, these objects and others in view, which will become apparent hereinafter, the invention provides a permanent-magnet rotor for electric motors according to claim 1 and a process for making such a rotor according to claims 11 and 16. Further characteristics and advantages of the rotor according to the invention will become apparent from the following detailed description of two preferred embodiments thereof, illustrated only by way of non-limitative example in the accompanying drawings, wherein: figure 1 is a perspective view of a first embodiment of the rotor according to the present invention; figure 2 is a perspective view of a magnet included in the rotor of figure 1; figure 3 is a partially sectional perspective view of a head flange included in the rotor of figure 1; figure 4 is a perspective view of the rotor of figure 1, during assembly prior to insertion in the mold; figure 5 is a perspective view of the rotor of figure 1 during insertion in the mold; figure 6 is a view of the step for the pouring of the rotor of figure 1; figure 7 is a sectional view of the rotor of figure 1 in the mold after pouring; figure 8 is a perspective exploded view of a second embodiment of the rotor with a thrust bearing which is associated therewith when installed; figure 9 is a sectional view of the unit of figure 8 in assembled condition; figure 10 is a view, taken from the thrust bearing end, of the rotor of figure 8 with part of the thrust bearing; figure 11 is an exploded perspective view of the components of the rotor of figure 8 prior to assembly in the mold; figure 12 is an enlarged-scale sectional view of a detail of the rotor of figure 8, taken from the thrust bearing end. With reference to the above figures 1 to 7, a permanent-magnet rotor for electric motors according to the invention comprises, in a first embodiment, two complementarily shaped semitubular permanent magnets 10 which are embedded, inside a casing 12 made of two-component epoxy resin. A core 12a embeds the ceramic shaft 11 and is made of the same material as the casing 12. The rotor is completed, at its heads, by respective retaining flange members 13 and 14 made of plastics which, as described in greater detail hereinafter, form elements for the correct positioning and centering of the magnets 10 and of the shaft 11 as well as elements for containing the resin during pouring. The first flange member 13 comprises a central ring 15 which is suitable to be crossed by the shaft 11 with a snug fit and is connected by means of radial spokes 16 to a perimetric ring 17 that extends axially and is suitable to retain the magnets 10. The second flange member 14 is instead constituted by a ring 18 with a U-shaped cross-section; the ends of the magnets 10 are inserted between the edges of said ring. An elastic lip 19 extends from the inner edge of the ring 18 and is suitable to retain the shaft 11 in position. The coupling between the lip 19 and the shaft 11 is such that it does not allow resin to pass during pouring. As regards the production process, a cup-shaped mold 20 is used which has a substantially cylindrical inner wall with a wide central bottom hole 21 required for the passage of the shaft 11 and for extraction. The pre-assembled rotor is placed in the mold 20, after applying a silicone release agent on the walls, by first inserting the second flange member 14, which has such an outer diameter as to sealingly couple to the bottom portion of the internal wall of said mold. The upper flange member 13 also externally forms a seal on the inner wall of the mold 20. It should be noted that in a downward region the couplings between the second flange member 14 and the shaft 11 and between said flange and the mold 20 close all passages. The subsequent operation consists in pouring a two-component epoxy resin into the empty regions 22 of the mold 20, through the empty spaces between the spokes 16 of the first flange member 13, and in allowing said resin to cure by reaction of the base with the catalyst. The empty regions 22 extend so as to form the casing 12 in which the magnets 10 and the shaft 11 are embedded. Finally, by applying adequate force to the second flange member 14, the rotor is extracted from the mold through the hole 21. Conveniently, in order to facilitate extraction from the mold, the inner wall of the mold 20 is given a slight taper. The high fluidity of the resin ensures complete filling of the empty regions in the mold 20 and thus complete embedding of the magnets 10 and of the shaft 11 in the casing which the resin finally forms. Pouring occurs at atmospheric pressure or at subatmospheric pressure (to avoid the forming of air bubbles in the resin) and therefore the magnets 10 and the shaft 11 are not subjected to intense stresses that might damage them. The resin furthermore has no effect on the magnetic field in which the rotor is placed during operation. With reference to the above mentioned figures 8 to 12, a permanent-magnet rotor for electric motors according to the invention comprises, in a second embodiment, two complementary shaped semitubular permanent magnets 110 which are contained, together with a ceramic shaft 111 axially inserted between them, in a substantially cup-shaped casing 112 which is obtained separately by injection-molding plastic material. Naturally, the bottom flange 113 of said cup-shaped casing 112 is centrally provided with a hole 114 for the passage of the ceramic shaft 111. It should also be noted that an axial tubular extension 115 again protrudes from the bottom flange 113 and retains the permanent magnets 110 in their correct position between said extension and the outer wall 116 of the casing 112. Again, the bottom flange 113 has an annular ridge 117 for supporting the magnets 110 in a raised position, forming empty passage regions. Again, the bottom flange 113 has through holes 118. As shown in the figures, the diameter of the ceramic shaft 111 leaves a certain clearance with respect to the magnets 110. The rotor is completed by a core 119 that mutually rigidly couples the magnets 110, the shaft 111 and the cup-shaped casing 112 and is obtained in a mold so as to fill the regions left empty by said parts, particularly the spacing 119a between the shaft 111 and the magnets 110, the space 119b between the bottom 113 and said magnets 110, and an upper disk-like portion 119c. The core 119 is made of injection-molded plastics or of poured two-component epoxy resin. The bottom 113 of the casing 112 is arrangeable at the end where the impeller, not illustrated in the drawings, is located; an internally toothed ring gear 120 rises coaxially to the shaft 111 from said bottom and is produced, in this case, by the molding-in-place operation, using the same material as the core 119, performed by virtue of the presence of the through holes 118 formed in the bottom 113. The ring gear can also be provided monolithically with the bottom of the casing (for example if the core is formed by pouring in a mold). Said ring gear 120 forms the seat 121 for a first component 122 of a thrust bearing, generally designated by the reference numeral 123, which is completed by a second component 124. The first component 122 comprises an annular ceramic element 125 which is externally provided with teeth and is enclosed in an elastomeric cap 126 provided with circumferential teeth and forming an elastic support in the seat 121 on the bottom 113. The second component 124 includes a metal bush 127 provided with an outer toroidal ring 128 made of elastomeric material. The two components 122 and 124 are assembled so that they are in end-to-end contact. At this point it should be stressed that the presence, in the rotor, of a cup-like casing 112 formed separately and then joined to the rest in a mold allows to form the outer wall 116 of said casing with very low thicknesses (a maximum thickness of 0.5 mm is optimum for the rotor) without causing the known problems involved in forming it directly in the mold together with the rest. The fact that the casing 112 is formed separately allows to use, for said casing, materials that better withstand high temperatures, for example for use with pumps for circulating hot water. The presence of the cup-like casing 112 eliminates production and utilization problems as regards both the core obtained by molding in place and the core possibly made of resin which does not deteriorate with use since it is inside the rotor and not exposed to damaging chemicals. It should also be particularly stressed that the support and the associated shaped seat on the impeller side for the thrust bearing 123 is obtained during the same molding-in-place operation. Consequently, the first component 122 of the thrust bearing is advantageously arranged in the best way with respect to the second component 124 against which it slides. The elastomeric cap 126 forms an intermediate means that can be adapted in order to damp vibrations. The shape of the seat is such that the assembly formed by the first component 122 and by the elastomeric cap 126 is located inside the ring 120 but is not actually retained in a fixed manner to the walls so that it can assume adequate planarity with respect to the second component 124. The only coupling, formed by the teeth of the ring gear 120 and of the cap 126, causes the component 122 to rotate by means of the rotor. In practice it has been observed that the intended aim and objects have been achieved in both embodiments. Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly such reference signs do not have any limiting effect on the scope of each element identified by way of example by such reference signs.

Claims: A permanent-magnet rotor for electric motors, comprising: at least two semitubular permanent magnets (10, 110) disposed around a shaft (11,111); a space defined between said semitubular permanent magnets (10, 110) and said shaft (11, 111); a core (12a, 119) arranged to snugly fill said space; an outer casing (12, 112) housing said at least two semitubular permanent magnets (10, 110); said semitubular permanent magnets (10, 110) and said shaft (11,111) being embedded within said core (12a,119) and said outer casing (12, 112), characterized in that a retaining flange member (13, 113) is provided for keeping in place said permanent magnets (10, 110) and said shaft (11, 111), said flange member comprising axial through holes distributed throughout the area of said flange member and of a shape adapted to permit molding material to be poured therethrough; in that said through holes are filled by the same molding material which forms at least said core and thus the flange member is made to be integral with said core; and that said core (12a, 112, 119, 119a) is made of a two-component epoxy resin material (16, 19). A permanent magnet rotor as claimed in claim 1, characterized in that said flange member comprises a first head flange (13) having a central ring (15) crossed by said shaft, said central ring being connected by means of radial spokes (16) to a perimetric ring (17) that extends axially and additionally retains said shaft and said magnets in position. A permanent magnet rotor as claimed in claim 1, in which the outer casing (12) is made of the same material and is integral with said core (12a). A rotor according to claim 1, characterized in that said outer casing (112, 113) is substantially cup-shaped and made of a plastic material which can be of a different nature with respect to the material forming said core (12), and is comprising a bottom flange (113) forming said retaining flange member. A rotor according to claim 4, characterized in that said cup-like element and its outer wall (112) is in engagement with said bottom flange (113) and that a tubular extension (115) protrudes from said bottom element (113) of said casing and forms at least part of said retaining flange member for keeping positioned said shaft. A rotor according to one or more of claims 4 and 5, characterized in that said core comprises: an annular portion (119a) between said shaft and said magnets; radial portions in the regions that separate said magnets; and two oppositely arranged disk-like regions, one (119b) located on said bottom of said casing and one (119c) located at the opposite end. A rotor according to one or more of claims 4 to 6, characterized in that ridges (117) protrude from the bottom of said substantially cup-shaped casing and keep said permanent magnets raised from said bottom. A rotor according to one or more of claims 4 to 7, characterized in that a ring gear (120) rises externally from said bottom element (113) of said casing, said ring gear forming a seat (121) for accommodating and supporting a component (122) of a thrust bearing (123). A rotor according to claim 8, characterized in that said component (122) comprises a ceramic annular element which is partially covered by an elastomeric cap (126) suitable to rest in said seat (121) formed externally on said bottom of said substantially cup-shaped casing. A rotor according to one or more of claims 4 to 9, characterized in that said ring gear has an internal set of teeth that meshes with an external set of teeth of a component of a thrust bearing, clearance being provided between said sets of teeth. A process for manufacturing a rotor as defined in claims 1-3 comprising the steps of providing at least two semitubular permanent magnets (10; 110); disposing said magnets about said shaft (11; 111) so as to provide a space between said shaft and said magnets; placing said magnets and said shaft in a cup-shaped mold (20; 112); providing one flange member (13) at one end of said cup-shaped mold for keeping in place said permanent magnets and said shaft; providing axial through holes in said flange member; pouring molding two component epoxy resin material trough said through holes so as to fill at least said space to form at least the core (12a, 112) of said rotor; and to fill at least said axial through holes of said flange member to render the flange integral with said core. A process according to claim 11, wherein said flange member (13) is inserted in the mold after the placing of said magnets and said shaft; the flange member comprising a central ring (15) for placing around said shaft (11,111) and a perimetric ring (17) connected to the central ring by means of radial spokes (16); the pouring occurring in the spaces between the radial spokes, which spaces form said axial through holes ; continuing the pouring until said (epoxy resin) material fills the space between the inner walls of the mold and the outer walls of said magnets so as to form said outer casing (12), molding material also filling the space formed between said spokes (16) of said upper flange. A process according to claim 11, comprising furthermore a second flange member (14) being inserted in the bottom of said mold for sealing thereof before the placing of said shaft (11, 111) and said magnets (10,110) therein; the second flange member (14) comprising a ring (18) with U-shaped cross section and a central elastic lip (19) forming a hole through which said shaft is sealingly inserted. A process for manufacturing a rotor according to claim 11, characterized in that a silicone release agent is applied on the inner walls of said mold prior to pouring, and in that the inner wall of said mold tapers slightly for extraction. A process according to claim 13, characterized in that said flanges form a seal, with their perimetric edges, on the inner wall of said mold. A process for manufacturing a rotor as defined in claim 4, comprising the steps of: forming a mold in the form of a cylindrical casing (112) and of a bottom flange (113) of independent plastic material so as to become a part of the rotor to be manufactured ; providing at least two semitubular permanent magnets (110) ; providing axial through holes (118) in said bottom flange; placing said shaft of said rotor in relative arrangement with respect to said permanent magnets so that a spacing (119a) is formed between said shaft and said permanent magnets; pouring molding, two component epoxy resin material in said casing to form the core (119) by filling with said molding material said spacing (119a); further filling with said poured molding material a space (119b) between the bottom flange and the magnets and filling said axial through holes (118) of said bottom flange to render said bottom flange integral with said core, and further forming with said poured molding material an upper disk-like portion (119c).

Inventors: MARIONI ELIO, MARIONI, ELIO
Applicants: ASKOLL SRL, ASKOLL S.P.A.
Application year: 1994
Main classification: H02K15
IPC codes: H02K1, H02K7, H02K15